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    Subject[PATCH 5.9 509/757] mm: fix a race during THP splitting
    Date
    From: Huang Ying <ying.huang@intel.com>

    [ Upstream commit c4f9c701f9b44299e6adbc58d1a4bb2c40383494 ]

    It is reported that the following bug is triggered if the HDD is used as
    swap device,

    [ 5758.157556] BUG: kernel NULL pointer dereference, address: 0000000000000007
    [ 5758.165331] #PF: supervisor write access in kernel mode
    [ 5758.171161] #PF: error_code(0x0002) - not-present page
    [ 5758.176894] PGD 0 P4D 0
    [ 5758.179721] Oops: 0002 [#1] SMP PTI
    [ 5758.183614] CPU: 10 PID: 316 Comm: kswapd1 Kdump: loaded Tainted: G S --------- --- 5.9.0-0.rc3.1.tst.el8.x86_64 #1
    [ 5758.196717] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS SE5C600.86B.02.01.0002.082220131453 08/22/2013
    [ 5758.208176] RIP: 0010:split_swap_cluster+0x47/0x60
    [ 5758.213522] Code: c1 e3 06 48 c1 eb 0f 48 8d 1c d8 48 89 df e8 d0 20 6a 00 80 63 07 fb 48 85 db 74 16 48 89 df c6 07 00 66 66 66 90 31 c0 5b c3 <80> 24 25 07 00 00 00 fb 31 c0 5b c3 b8 f0 ff ff ff 5b c3 66 0f 1f
    [ 5758.234478] RSP: 0018:ffffb147442d7af0 EFLAGS: 00010246
    [ 5758.240309] RAX: 0000000000000000 RBX: 000000000014b217 RCX: ffffb14779fd9000
    [ 5758.248281] RDX: 000000000014b217 RSI: ffff9c52f2ab1400 RDI: 000000000014b217
    [ 5758.256246] RBP: ffffe00c51168080 R08: ffffe00c5116fe08 R09: ffff9c52fffd3000
    [ 5758.264208] R10: ffffe00c511537c8 R11: ffff9c52fffd3c90 R12: 0000000000000000
    [ 5758.272172] R13: ffffe00c51170000 R14: ffffe00c51170000 R15: ffffe00c51168040
    [ 5758.280134] FS: 0000000000000000(0000) GS:ffff9c52f2a80000(0000) knlGS:0000000000000000
    [ 5758.289163] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [ 5758.295575] CR2: 0000000000000007 CR3: 0000000022a0e003 CR4: 00000000000606e0
    [ 5758.303538] Call Trace:
    [ 5758.306273] split_huge_page_to_list+0x88b/0x950
    [ 5758.311433] deferred_split_scan+0x1ca/0x310
    [ 5758.316202] do_shrink_slab+0x12c/0x2a0
    [ 5758.320491] shrink_slab+0x20f/0x2c0
    [ 5758.324482] shrink_node+0x240/0x6c0
    [ 5758.328469] balance_pgdat+0x2d1/0x550
    [ 5758.332652] kswapd+0x201/0x3c0
    [ 5758.336157] ? finish_wait+0x80/0x80
    [ 5758.340147] ? balance_pgdat+0x550/0x550
    [ 5758.344525] kthread+0x114/0x130
    [ 5758.348126] ? kthread_park+0x80/0x80
    [ 5758.352214] ret_from_fork+0x22/0x30
    [ 5758.356203] Modules linked in: fuse zram rfkill sunrpc intel_rapl_msr intel_rapl_common sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp mgag200 iTCO_wdt crct10dif_pclmul iTCO_vendor_support drm_kms_helper crc32_pclmul ghash_clmulni_intel syscopyarea sysfillrect sysimgblt fb_sys_fops cec rapl joydev intel_cstate ipmi_si ipmi_devintf drm intel_uncore i2c_i801 ipmi_msghandler pcspkr lpc_ich mei_me i2c_smbus mei ioatdma ip_tables xfs libcrc32c sr_mod sd_mod cdrom t10_pi sg igb ahci libahci i2c_algo_bit crc32c_intel libata dca wmi dm_mirror dm_region_hash dm_log dm_mod
    [ 5758.412673] CR2: 0000000000000007
    [ 0.000000] Linux version 5.9.0-0.rc3.1.tst.el8.x86_64 (mockbuild@x86-vm-15.build.eng.bos.redhat.com) (gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5), GNU ld version 2.30-79.el8) #1 SMP Wed Sep 9 16:03:34 EDT 2020

    After further digging it's found that the following race condition exists in the
    original implementation,

    CPU1 CPU2
    ---- ----
    deferred_split_scan()
    split_huge_page(page) /* page isn't compound head */
    split_huge_page_to_list(page, NULL)
    __split_huge_page(page, )
    ClearPageCompound(head)
    /* unlock all subpages except page (not head) */
    add_to_swap(head) /* not THP */
    get_swap_page(head)
    add_to_swap_cache(head, )
    SetPageSwapCache(head)
    if PageSwapCache(head)
    split_swap_cluster(/* swap entry of head */)
    /* Deref sis->cluster_info: NULL accessing! */

    So, in split_huge_page_to_list(), PageSwapCache() is called for the already
    split and unlocked "head", which may be added to swap cache in another CPU. So
    split_swap_cluster() may be called wrongly.

    To fix the race, the call to split_swap_cluster() is moved to
    __split_huge_page() before all subpages are unlocked. So that the
    PageSwapCache() is stable.

    Fixes: 59807685a7e77 ("mm, THP, swap: support splitting THP for THP swap out")
    Reported-by: Rafael Aquini <aquini@redhat.com>
    Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Tested-by: Rafael Aquini <aquini@redhat.com>
    Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Matthew Wilcox <willy@infradead.org>
    Link: https://lkml.kernel.org/r/20201009073647.1531083-1-ying.huang@intel.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/huge_memory.c | 13 +++++++------
    1 file changed, 7 insertions(+), 6 deletions(-)

    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index d37e205d3eae7..18a6f8c8b2844 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -2473,6 +2473,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,

    remap_page(head, nr);

    + if (PageSwapCache(head)) {
    + swp_entry_t entry = { .val = page_private(head) };
    +
    + split_swap_cluster(entry);
    + }
    +
    for (i = 0; i < nr; i++) {
    struct page *subpage = head + i;
    if (subpage == page)
    @@ -2707,12 +2713,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
    }

    __split_huge_page(page, list, end, flags);
    - if (PageSwapCache(head)) {
    - swp_entry_t entry = { .val = page_private(head) };
    -
    - ret = split_swap_cluster(entry);
    - } else
    - ret = 0;
    + ret = 0;
    } else {
    if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
    pr_alert("total_mapcount: %u, page_count(): %u\n",
    --
    2.25.1


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